1. Field of the Invention
This invention relates to polycarbonate copolymers and methods of their molding, and more particularly, to copolycarbonates of thiodiphenol having high melt flow rates and excellent physical properties.
2. Description of the Prior Art
Polycarbonates derived from reactions involving organic dihydroxy compounds and carbonic acid derivatives have found extensive commercial application because of their excellent mechanical and physical properties. These thermoplastic polymers are particularly suited for the manufacture of molded articles where impact strength, rigidity, toughness, thermal and dimensional stability as well as excellent electrical properties are required.
However, one deficiency of polycarbonate when used in molded articles is their low critical thickness.
It is known that polycarbonate plastics exhibit high notched Izod (ASTM Test D-256) impact values. This value, however, is dependent upon the thickness of the test specimen. Typical notched Izod impact values for a 1/8" specimen are about 10 to 16 ft.-lbs. per inch. These high Izod values result because specimens of 1/8" thickness are thinner than the critical thickness of the polymer, and are associated with hinged or ductile breaks. On the other hand, a 1/4" specimen exhibits a clean or brittle break with a notched Izod impact value of only about 2.5 ft.-lbs. per inch. The 1/4" specimens are said to be above the critical thickness of the polymer. "Critical thickness" has been defined as the thickness at which a discontinuity in the graphical representation of Izod impact strength vs. specimen's thickness occurs. In other words, it is the thickness at which a transition from a brittle to a ductile failure takes place. Thus, a standard impact specimen of polycarbonate polymer thicker than the critical thickness exhibits brittle breaks, and those thinner than the critical thickness exhibit hinged or ductile breaks. Further, the critical thickness of a polycarbonate based on bisphenol A with a melt flow of 3 to 6 g/10 min. at 300.degree. C. (ASTM D-1238) has a critical thickness of about 225 mils.
The melt flow rate of a polycarbonate resin is a measure generally of both molecular weight and of processability. A low melt flow rate, i.e., high viscosity, indicates processing difficulties. On the other hand, higher viscosity is also indicative of a high molecular weight which is generally associated with the exceptional chemical and mechanical properties of polycarbonates.
It was thus generally recognized that the desirable properties of polycarbonate are not associated with easy-to-process resins. It has been further recognized that in molding thin parts i.e., less than 20 mils, or in using molds of complicated patterns, a high viscosity polycarbonate cools down and stops flowing before completely filling the mold.
In accordance with the present invention a copolycarbonate is provided wherein combined are a high melt flow rate and excellent mechanical, chemical and physical properties which polycarbonates are noted for. Further disclosed is a method of injection molding a copolycarbonate characterized by its critical thickness being in excess of 200 mils, at less than 500.degree. F. and preferably lower than 485.degree. F. Additionally disclosed is a method for molding an impact resistant copolycarbonate resin characterized by its high critical thickness, into molds having sections thereof thinner than 20 mils and preferably thinner than 5 mils.